Acetonitrile on the current market is mostly prepared by purifying crude acetonitrile recovered as a by-product in the production of acrylonitrile or methacrylonitrile comprising catalytic ammoxydation of propylene or isobutene with ammonia and molecular oxygen Since the by-produced acetonitrile contains many impurities, various processes of purification have conventionally been proposed as follows.
Known processes for removing allyl alcohol from crude acetonitrile include reaction with sulfuric acid followed by distillation (JP-A-51-23218, the term "JP-A as used herein means an "unexamined published Japanese patent application"), extractive distillation in the presence of water (JP-A-55-143949), and reaction with an aqueous solution of an alkali metal or alkaline earth metal hypochlorite (JP-A-51-32518). Known processes for removing oxazole from crude acetonitrile include reaction with molecular chlorine followed by separation (JP-A-59-10556) and extractive distillation in the presence of water (JP-A-55-143950).
Known processes for purifying crude acetonitrile include reaction with a basic compound followed by distillation (JP-A-56-5449) and distillation using three distillation columns (JP-A-58-124751) In short, the by-product acetonitrile contains allyl alcohol, oxazole, acrylonitrile, etc. as main impurities and, in order to remove these impurities it has been necessary to combine some of these processes proposed to date, making the whole purification process very complicated. Moreover, the present inventors verified by experiments that each of these known processes fails to afford acetonitrile having an absorbance of not more than 0.05 at 200 to 400 nm.
East German Patent DD217212A1 discloses an excellent purification process comprising contacting crude acetonitrile with ozone to simultaneously decompose allyl alcohol, oxazole, acrolein, etc. and separating purified acetonitrile by distillation However, as a result of verification experiments on this process, the present inventors revealed that the process really succeeds in reducing the absorbance at 200 to 400 nm to some extent but not to 0.05 or less.
Therefore, commercially available acetonitrile is unsatisfactory as a mobile phase solvent for liquid chromatography, particularly HPLC due to its high UV (200 to 400 nm) absorbance. It has thus been demanded to develop an industrially beneficial process for preparing acetonitrile having sufficient purity for use as a mobile phase solvent for liquid chromatography
The UV absorption of crude acetonitrile at 200 to 400 nm is chiefly ascribed to the double bonds of impurities due to the production process. For example, compounds having a carbon-to-carbon double bond, a carbonyl group (C.dbd.O), a carboxyl group (COOH), an aldehyde group (CHO), a carbon-to-nitrogen double bond, a nitroso group (N.dbd.O), a nitro group (NO.sub.2) or a like bond or functional group,exhibit absorption in that wavelength region. Examples of compounds with such a bond or a functional group are allyl alcohol, oxazole, acrylonitrile, methacrylonitrile, cis- and trans-crotononitrile, acrylic acid, methyl acrylate, methacrylic acid, methyl methacrylate, acetic acid, acrolein, methacrolein, and acetone. It is very difficult to remove these compounds to reduce the absorbance at 200 to 400 nm to 0.05 or less. More specifically, in order to reduce the absorbance of acetonitrile at 200 nm to 0.05 or less, the acetonitrile must have its allyl alcohol content reduced to 1.5 ppm or less; its oxazole content to 0.8 ppm or less; its acrylonitrile content to 0.2 ppm or less; its methacrylonitrile content to 0.2 ppm or less; its cis-crotononitrile content to 0.2 ppm or less; its acrylic acid content to 0.2 ppm or less; its methyl acrylate content to 0.2 ppm or less; and its acetic acid content to 30 ppm or less. The same strict requirement applies to other impurities. Because crude acetonitrile usually contains no less than two of these compounds, the critical content of each compound would actually be lower than the above value. Accordingly, obtaining highly purified acetonitrile admittedly requires high techniques.